manual/user guide: high-level restructuring

manual/user guide: high-level restructuring

This patch performs some additional restructuring of the manual,
specifically in the User Guide. In detail:

- Rename 'Daily use' to 'General Buildroot usage'
- Move chapters 'make tips', 'Eclipse integration', and 'Advanced usage' as
  sections under the 'General Buildroot usage' chapter.
- Rename 'Details on Buildroot configuration' into 'Buildroot configuration'
- Rework the 'Customization' section as follows:
  - Move the short section on debugging the external toolchain wrapper into
    the rest of the explanation on external toolchains.
  - Remove the now redundant section on toolchains, as this is already
    explained in much more detail in the 'Buildroot configuration' chapter.
  - Move the sections on busybox/uclibc/kernel configuration from chapter
    'Customization' into a separate chapter 'Configuration of other
    components'.
  - Rename the remaining part of the original 'Customization' chapter into
    'Project-specific customization' and fold it together with the next
    chapter 'Storing the configuration'
- Remove the chapter 'Going further in Buildroot innards' thanks to:
  - Moving the chapter 'How Buildroot works' to the Developer guide.
  - Moving the 'Advanced Buildroot usage' section to the 'General Buildroot
    usage' chapter.
- Remove the chapter 'Hacking Buildroot' by:
  - Adding a reference to adding packages to the 'Project-specific
    customizations' chapter
  - Leaving out the explicit reference to creating board support, as this is
    part of the previous chapter already, so an extra reference is
    redundant.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>